Hi Bowen,

Thanks for your quick reply. 

Sorry I used the wrong term there, there it is a maintenance window rather than 
an outage. This is a key system and the vital nature of it means that the 
customer is rightly very risk adverse, so we will only even get permission to 
upgrade one DC per night via a rolling upgrade, meaning this will always be 
over more than a week. 

So we can’t shorten the time the cluster is in mixed mode, but I am concerned 
about having a schema mismatch for this long time. Should I be concerned, or 
have others upgraded in a similar way?



> On 24 Apr 2024, at 17:02, Bowen Song via user <user@cassandra.apache.org> 
> wrote:
> Hi Paul,
> You don't need to plan for or introduce an outage for a rolling upgrade, 
> which is the preferred route. It isn't advisable to take down an entire DC to 
> do upgrade.
> You should aim to complete upgrading the entire cluster and finish a full 
> repair within the shortest gc_grace_seconds (default to 10 days) of all 
> tables. Failing to do that may cause data resurrections.
> During the rolling upgrade, you should not run repair or any DDL query (such 
> You don't need to do the rolling upgrade node by node. You can do it rack by 
> rack. Stopping all nodes in a single rack and upgrade them concurrently is 
> much faster. The number of nodes doesn't matter that much to the time 
> required to complete a rolling upgrade, it's the number of DCs and racks 
> matter.
> Cheers,
> Bowen
> On 24/04/2024 16:16, Paul Chandler wrote:
>> Hi all,
>> We have some large clusters ( 1000+  nodes ), these are across multiple 
>> datacenters.
>> When we perform upgrades we would normally upgrade a DC at a time during a 
>> planned outage for one DC. This means that a cluster might be in a mixed 
>> mode with multiple versions for a week or 2.
>> We have noticed that during our testing that upgrading to 4.1 causes a 
>> schema mismatch due to the new tables added into the system keyspace.
>> Is this going to be an issue if this schema mismatch lasts for maybe several 
>> weeks? I assume that running any DDL during that time would be a bad idea, 
>> is there any other issues to look out for?
>> Thanks
>> Paul Chandler

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